1. Field of the Invention
The present invention relates to a resin-coated metal can. More specifically, the invention relates to a seamless can formed of a resin-coated metal plate which features excellent workability, adhesiveness, corrosion resistance and shock resistance, and prevents the oligomer component of a polyester from eluting out.
2. Prior Art
Side-seamless cans have heretofore been produced by draw-ironing a resin-coated metal plate that is obtained by applying an organic coating onto a metal blank. However, the organic coating on the inner surface is subject to be damaged by the tools. In the portions where the coating is damaged, further, the metal is exposed actually or latently, and starts metal picking up or corrodes from these portions. In producing the seamless cans, further, there occurs a plastic flow causing the size to increase in the direction of height of the can but causing the size to shrink in the circumferential direction of the can. When the plastic flow occurs, the adhesive force decreases between the metallic surface and the organic coating and, besides, the adhesive force between the two decreases with the passage of time due to residual distortion in the organic coating. This tendency becomes conspicuous particularly when the content is hot-packaged in the can and when the canned content is heat-sterilized at a low temperature or at a high temperature.
A draw-formed can which uses a laminated member on which a films of a polyethylene terephthalate or a polyethylene terephthalate/isophthalate is laminated as an organic coating, exhibits excellent barrier property against corrosive components and excellent workability accompanied, however, by a problem of a decrease in the shock resistance of the film covering the can and, particularly, a markedly deceased resistance against being dented when the container after worked is packaged with the content and is left to stand.
In order to solve this problem, Japanese Unexamined Patent Publication (Kokai) No. 108650/1995 teaches a draw-formed container made of a metal plate on which is formed a laminated film which comprises:
a surface layer of a high-molecular oriented copolymerized polyester (A) derived from a diol component and from acid components of 85 to 97% of terephthalic acid and 3 to 15% of isophthalic acid; and
a lower layer of a low-molecular oriented polymerized polyester (B) derived from a diol component and from acid components of 84.5 to 96.5% of terephthalic acid and 3.5 to 15.5% of isophthalic acid, the isophthalic acid being contained in an amount larger than the amount contained in the copolymerized polyester (A) of the surface layer.
Further, Japanese Unexamined Patent Publication (Kokai) No. 178485/1995 discloses a two-piece can obtained by draw-ironing or stretch-drawing and having an organic resin coating of which the main component is a polyester resin containing oriented crystals, the polyester resin layer having an inherent viscosity of not smaller than 0.60, and a parameter A representing the axial orientation degree of the oriented crystals in the direction of height of the can being Axe2x89xa70.40, and a parameter B representing the axial orientation degree of crystals axially oriented in the direction of height of the can being Bxe2x89xa70.00.
The resin-coated metal plate forming the former draw-formed container exhibits flavor-retaining property, shock resistance and, particularly, resistance against being dented yet maintaining fairly excellent workability and adhesiveness. However, this oriented film laminated material cannot withstand high degree of drawing or draw-forming.
The latter metal can formed by stretch working and/or ironing has a decreased thickness in the can wall to decrease the weight and cost. As the thickness of the can wall becomes small, however, a high degree of workability is required for the resin applied to the metal plate, and it becomes necessary to use a resin-coated metal plate capable of withstanding a high degree of working.
In general, further, the can worked to a high degree builds up distortion to a considerable degree. Therefore, the resin-coated metal plate can be used for the vacuumed cans that build up distortion little but cannot be used for the pressured cans that build up distortion much.
When subjected to lowly acidic conditions such as containing coffee and stored in a hot vending machine or when subjected to severe conditions such as high-temperature wet heated condition like retort sterilization, even the resin-coated metal plate which is satisfactory under ordinary conditions loses corrosion resistance and resistance against being dented, permitting the oligomer component of polyester to elute out and causing the content to become turbid.
In recent years, in particular, a high-temperature retort has been desired for rational and efficient retort sterilization. In the retort at a high temperature, low-molecular components contained in the film migrate in increased amounts into the content. Among the low-molecular components, further, those components having relatively high molecular weights and which are very little soluble in an aqueous solution tend to be extracted markedly. Though the amount that migrates into the content may be very smaller than limit amounts stipulated under the Regulations of MITI, Japan and U.S. Food and Drug Administration Regulations, the components having relatively large high molecular weights migrated into the content are coagulated to form particles of large sizes resulting in the occurrence of turbidity when treated at high temperatures or preserved for extended periods of time, which is not psychologically desirable.
It is therefore an object of the present invention to provide a resin-coated seamless can that satisfies can properties such as high degree of workability, adhesion of film, resistance against high temperature and wet heated condition, corrosion resistance and resistance against being dented. Another object of the present invention is to provide a resin-coated seamless can which prevents low-molecular components from eluting into the content when treated at high temperatures and preserved for extended periods of time, which does not cause turbidity, and exhibits excellent flavor-retaining property.
According to the present invention, there is provided a resin-coated seamless can formed of a resin-coated metal plate having substantially unoriented two layers including a surface layer and a lower layer serving as a resin layer on the inner surface side of the can, wherein the surface layer comprises a polyethylene terephthalate/isophthalate containing an isophthalic acid in an amount of from 3 to 13 mol % and having an intrinsic viscosity [xcex7] of not smaller than 0.7, and the lower layer comprises a polyethylene terephthalate/isophthalate containing the isophthalic acid in an amount of from 8 to 25 mol %, which is larger than that of the surface layer, and having an intrinsic viscosity [xcex7] of not smaller than 0.7, the resin layer of the upper part of the can on the inner surface side satisfying the following formula (1),
Cs greater than Caxe2x80x83xe2x80x83(1) 
where Cs=Is973/Is795, Ca=Ia973/Ia795, Is973 is an infrared absorption intensity at 973 cmxe2x88x921 measured for the surface layer, Is795 is an infrared absorption intensity at 795 cmxe2x88x921 measured for the surface layer, Ia973 is an infrared absorption intensity at 973 cmxe2x88x921 measured for the lower layer and Ia795 is an infrared absorption intensity at 795 cmxe2x88x921 measured for the lower layer.
In the resin-coated seamless can, it is desired that the coefficient of surface orientation of the resin layer on the inner surface side on the bottom of the can is not larger than 0.05.
In the present invention, it is desired that the half-crystallization time of the lower layer at 160xc2x0 C. is not shorter than 100 seconds, and that the terephthalate/isophthalate resin of the surface layer and of the lower layer are produced by the solid-phase polymerization method.
It is further desired that the surface layer (A) and the lower layer (B) have a thickness ratio of,
A:B=1:15 to 15:1 
and that the surface layer and the lower layer are laminated by the lamination of multi-layer cast films or by the co-extruded coating, and have not been substantially oriented in a state of the resin-coated metal plate before being formed into a can.
It is desired that the seamless can of the invention is obtained by drawing or stretch-drawing the resin-coated metal plate, or by ironing the resin-coated metal plate.
In the seamless can, further, the surface layer and the lower layer of the upper part of the can have an amorphous parameter (F) defined by the following formula (2),
F=xe2x88x920.95X100xe2x88x920.52Xxe2x88x92105+8.4xe2x80x83xe2x80x83(2) 
wherein X100 is a maximum value of normalized strength of X-ray diffraction gotten by X-ray diffraction pole figure diagram of (100) pole, and Xxe2x88x92105 is a maximum value of normalized strength of X-ray diffraction gotten by X-ray diffraction pole figure diagram of (xe2x88x92105) pole, of not smaller than 3.